1. Field of the Invention
This invention relates to the field of using magnetic sensors to measure torque applied to a rotatable shaft. More particularly, the invention relates to the particular placement of magnetic torque sensors in relation to magnetic field bands to reduce the axial movement error in a torque system.
2. Description of the Related Art
In control of systems having rotatable drive shafts, torque and speed are the fundamental parameters of interest. The sensing and measurement of torque in an accurate, reliable, and inexpensive manner has been a primary objective of workers for several decades. Examples of applications in which inexpensive torque sensing devices are used include electric power steering systems, rotating shafts in machinery, and others. In such environments, the torque sensing devices are typically required to make low-error continuous torque measurements over extended time periods, despite severe operating conditions.
FIGS. 1 and 1A show a typical system for measuring the torque applied to a rotatable shaft using non-contact magnetic field sensors. The sensors are positioned approximately directly over the peak axial strength area of each band of magnetic field. However, the shaft may have axial movement due to its floating design or the force applied. This configuration is very susceptible to any axial, or left and right, physical movement of the shaft. Since the sum of the sensor readings indicates the amount of torque measured, if the fields change at the sensor locations, due to movement by the shaft or ring, then both sensors will be at a point of lower axial field, as shown in FIG. 2. The signal becomes weaker and is interpreted as a loss of torque, when the actual field and torque is unchanged. This causes a greater tendency for error in the torque reading.
Accordingly, it is desirable to create a more accurate configuration for the magnetic sensors to reduce the torque measurement error due to the physical axial movement of the shaft in the system.